You slowly learn what's good and what's bad. I really started to stay away from using obscure facts that I thought other people would know. There are things you know that no one else in the world knows and you have to realize that.
There is some math involved. Words have certain patterns and proportions and a lot of it is learning the mathematical nature of words. The rest is just having a sense for words.
It's something I care about for some silly reason.